That Whispernet works because the cost can easily be built into the device, or the books — ebooks aren't large downloads and therefore it's easy to set up cheap deals with carriers. After all, Kindle users won't be data hogs or cause network congestion (I say this from the view of a carrier, not that network congestion is acceptable).

Music & videos however add up fast.

Kindle delivery is baked into the rake Amazon takes off the top, along with their 30%.

If you opt or prince into the 35% royalty model (i.e., 65% to AMZN) then they are graciously willing to absorb that cost themselves.